1. Field of the Invention
The present invention relates to an electroless copper plating solution and to a high-frequency electronic component having a copper plating film formed by using the electroless copper plating solution.
2. Description of the Related Art
When copper plating films used as high-frequency conductors are formed, an alkaline electroless copper plating solution, which is an aqueous solution containing copper ions, formaldehyde used as a reducing agent, and ethylenediamine tetraacetic acid (EDTA) used as a complexing agent, has generally been used. In addition, a copper plating film approximately 2 to 4 xcexcm thick formed by using an electroless copper plating solution has been used as an electrode in high-frequency electronic components, such as a dielectric coaxial resonator used in the region of 800 to 2,000 MHz or a millimeter wave resonator used in the region of 20 to 30 GHz.
In this connection, the superior conductivity of copper plating films has been important in high-frequency electronic components, for example, dielectric resonators, and in addition, it has also been important that copper plating films do not develop blisters and have superior adhesion to ceramics. In addition, when a conventional electroless copper plating solution containing EDTA is used, the surfaces of a ceramic substrate are strongly etched beforehand using an etching agent containing hydrogen fluoride so as to ensure the adhesion of the copper plating film by fully exploiting the anchor effect obtained by roughening ceramic surfaces.
However, when the roughness of the ceramic surface is increased by strong etching, the high-frequency conductivity is decreased, and in addition to a decrease in the Q value (resonance sharpness), the so-called Qc value, of the copper plating film itself used as an electrode, the Q value of a dielectric resonator, that is, the so-called Qo value, is also decreased. In order to avoid these inconveniences, it has been considered that even when a conventional electroless copper plating solution containing EDTA is used, the etching should be mildly performed so that the roughness of the ceramic surface is decreased.
However, when the roughness of a ceramic surface is decreased and plating is then performed, blisters are developed on the copper plating film, and hence, superior adhesion of the copper plating film to the ceramic substrate cannot be reliably obtained. It has been believed that the reason this phenomenon occurs is that since hydrogen is violently generated due to the decomposition of formaldehyde, which is used as a reducing agent, contained in the electroless copper plating solution, the anchor effect is decreased by the generated hydrogen which interferes with the deposition of copper on the ceramic substrate having low surfaces roughness.
In consideration of the problems described above, the present invention was made, and an object of the present invention is to provide an electroless copper plating solution which can form a copper plating film having superior high-frequency conductivity and superior adhesion to a smooth ceramic surface having low roughness. In addition, the present invention also provides a high-frequency electronic component having a high Qo value formed by using this electroless copper plating solution.
In accordance with one aspect of the present invention, there is provided an electroless copper plating solution comprising copper ions; nickel ions; formaldehyde or a derivative thereof; and tartaric acid or a salt thereof. In addition, the ratio of the content of the nickel ions to that of the copper ions on a molar basis in the electroless copper plating solution is preferably in the range of about 0.0001 to 0.015 and more preferably about 0.0001 to 0.01.
When an electroless copper plating solution contains tartaric acid or a salt thereof as a complexing agent and does not contain nickel, plating stops before precipitation of copper sufficiently proceeds, and as a result, it has not been possible to form a copper plating film having a thickness of about 2 xcexcm or more in practice. However, with the electroless copper plating solution of the present invention, which contains nickel, the nickel functions as a catalyst for precipitation to allow the copper to continuously precipitate, and hence, the thick copper film generally used for high-frequency conductors can be easily formed.
In addition, when the electroless copper plating solution of the present invention is used in the case in which plating is performed on a ceramic substrate having low surface roughness, the amount of hydrogen generated by decomposition of formaldehyde caused by the effect of the added nickel together with tartaric acid functioning as a complexing agent is decreased. Accordingly, the copper easily precipitates on the low surface roughness ceramic surface and blisters are not developed in the copper plating film, whereby superior adhesion of the copper plating film to the ceramic substrate can be reliably obtained. In addition, since the content of the nickel is small, conductivity equivalent to that of pure copper can be obtained. Furthermore, the nickel added in a small amount also serves to improve the adhesion of the copper plating film to the ceramic substrate in combination with oxygen.
In accordance with another aspect of the present invention, there is provided a high-frequency electronic component comprising a copper plating film formed by plating using the electroless copper plating solution described above. In addition, there is provided a high-frequency electronic component in accordance with another aspect of the present invention, comprising a dielectric ceramic and a metal film formed on a surface of the dielectric ceramic, wherein the metal film is primarily composed of copper containing at least nickel, and the ratio of the content of the nickel to that of the copper on a molar basis is in the range of about 0.0001 to 0.015.
A high-frequency electronic component, such as a dielectric resonator, formed as described above has a copper plating film which is preferably used as a high-frequency conductor, that is, an electrode having a thick copper plating film. Accordingly, superior high-frequency conductivity and a high Qo value can be advantageously obtained.